It will be useful to provide an Enhanced Observed Time Difference (E-OTD) functionality in the U.S. Time Division Multiple Access (TDMA) wireless standard known as ANSI-136. The E-OTD would be used by the mobile station and the wireless network when making measurements of neighboring cell base stations, and is thus important for determining the location of the mobile station.
In practice, the E-OTD method for mobile station neighbor base station measurements is based on mobile station-made measurements of observed time differences between a reference base station and at least two neighbor base stations. A wireless network element known as a Location Measurement Unit (LMU) is associated with all or substantially all base transceiver stations, also referred to as base stations. The LMU measures the real time difference (RTD) between the transmission timing of neighbor base stations and the reference base station. When the OTD and the RTD are known the Geometric Time Difference (GTD) can be calculated as: GTD=OTD-RTD.
While the use of the E-OTD has been standardized in Global System for Mobile Communications (GSM) wireless networks, at present there is no standard available to provide E-OTD functionality for ANSI-136 networks. A simple incorporation of the E-OTD GSM functionality into ANSI-136 is not practical, as a number of differences exist between the air interfaces of GSM and ANSI-136. For example, ANSI-136 does not support the Base Station Identity Code (BSIC) that is supported by GSM. Other significant differences exist as well.
As a further example, in the ANSI-136 network the mobile station (e.g., a cellular telephone or personal communicator) receives from an Extended Broadcast Control Channel (E-BCCH) a neighbor list of base stations. The neighbor list includes a frequency channel number and a Digital Voice Color Code (DVCC) for a frequency channel that the mobile station is expected to measure. The DVCC is used to identify the base station that transmits the frequency channel, and each frequency channel transmitted by a given base station will typically include a different DVCC, and the same frequency transmitted by two different base stations will ideally also have different DVCCs. A problem arises in that the mobile station must ensure that the channel that it is measuring is the correct channel, and not a channel being received from a base station in some other cell. This could be done by relying on a proper network design, or by receiving the Fast Broadcast Control Channel (F-BCCH) on the neighbor cell Digital Control Channel (DCCH) and decoding the DVCC field that is a part of the F-BCCH. The mobile station could then verify that the decoded DVCC is the same as was broadcast in the neighbor cell message on the serving cell's DCCH. Unfortunately, the reception of the neighbor cell F-BCCH requires a period of time that is greater than one second, which would result in unacceptable delays for the E-OTD measurement process. As such, a need exists to quickly and reliably identify a channel to be measured.
A further problem arises in regard to the actual measurement report from the mobile station to the Serving Mobile Location Center (SMLC) of the wireless network, as the mobile station must identify the channel that it has measured. Simply providing a channel number is not sufficient as additional information, such as channel number and the channel's DVCC, would provide better information. Even further information, such as the channel number, DVCC and the hyperband information, would be preferred.
As a point of reference, in the GSM system the following alternative procedures can be used by the mobile station for identifying a neighbor cell that is measured:
1. the cell identity is conveyed by using C1, and the Location Area Code (LAC) is the same as the current serving base station;
2. the cell identity is conveyed using the 51 Multiframe offset and the BCCH carrier;
3. the cell identity is conveyed using an index that refers to the Base Transceiver Station (BTS) listed in the Measure Position Request component (the indicated reference BTS is 1); or
4. the cell identity is conveyed using an index that refers to the BTS listed in the BCCH allocation list (System Information Neighbor Lists) of the serving BTS.
The first three techniques are not applicable to the ANSI-136 system. The fourth technique might be applicable, but it is network implementation specific, and thus may not be applicable universally across all ANSI-136 networks. This would cause a problem when the mobile station roams between networks.